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• TAB b GEOLOGY AND OVERBURDEN ASSESSMENT Geologic Description Regional Structure. The Toast Permit Area is situated in the rugged Yilliams Fork Mountains which ere within the southeastern synclinal portion of the Sand (lash structural basin of northern Colorado and central Nyoming (Bass et al., 1955). In northern Colorado, the synclinal portion of the Sand wash basin is bordered on the southwest by the Axial Basin anticline, and on the east by the Park Range anticline. Numerous folds modify the synclinal structure of the region, the largest of which is the Tow Creek anticline Located approximately six miles northeast of the permit area (Figure 1). The Tow Creek anticline as well as the area that lies approximately 11 miles to the south of it are heavily faulted. Approximately five to ten miles southeast of the Yoast permit site are numerous faults trending northeast to southwest. Regional Stratigraph v. The exposed rock sequence in the region consists primarily of about 13,500 feet of Late Cretaceous, Tertiary, and puaternery age sedimentary deposits. Some intrusive and extrusive igneous rocks that are probably Miocene in age (Bass e[ al ., 1955) occur regionally. The largest regional occurrence of extrusive rocks occurs in the Flat Tops about 12 miles south of the Yoast permit area (Figure 1). The oldest rock formation outcropping in the region is the Mancos shale of Late Cretaceous age. The nearest outcrop is immediately southwest of the permit boundary along Sage Creek and the Sage Creek anticline (Figure 1). The Mancos shale is about 4,900 .feet thick and is conformable with the overlying Meseverde group and the underlying Dakota sandstone (Bess et al., 1955). The Mancos shale is predominantly a dark gray marine shale with interbedded layers of sandstone near the top and interbedded layers of sandstone and limestone near the base. There is a sandstone layer about 50 feet in thickness situated approximately 300 feet above the base of the formation which is thought to be equivalent to part of the Frontier sandstone (Bass et al., 1955). Above the Mancos Shale lies the Mesaverde Group, also of late Cretaceous age. This Broup consists of tuo formations, the older being the Iles Formation and the younger being the Yilliams Fork Formation (Figure • 1). Together these two formations represent about 2,550 to 3,550 feet of sandstone interbedded with shales, sandy shales, end coal (Bass et el., 1955). 1